diff --git a/src/libraries/System.Memory/src/Resources/Strings.resx b/src/libraries/System.Memory/src/Resources/Strings.resx
index 90489cb74c5898..0c13320a1e0e01 100644
--- a/src/libraries/System.Memory/src/Resources/Strings.resx
+++ b/src/libraries/System.Memory/src/Resources/Strings.resx
@@ -147,13 +147,10 @@
   <data name="BufferMaximumSizeExceeded" xml:space="preserve">
     <value>Cannot allocate a buffer of size {0}.</value>
   </data>
+  <data name="NotSupported_UnseekableStream" xml:space="preserve">
+    <value>Stream does not support seeking.</value>
+  </data>
   <data name="NotSupported_UnwritableStream" xml:space="preserve">
     <value>Stream does not support writing.</value>
   </data>
-  <data name="IO_SeekBeforeBegin" xml:space="preserve">
-    <value>An attempt was made to move the position before the beginning of the stream.</value>
-  </data>
-  <data name="Argument_InvalidSeekOrigin" xml:space="preserve">
-    <value>Invalid seek origin.</value>
-  </data>
 </root>
\ No newline at end of file
diff --git a/src/libraries/System.Memory/src/System/Buffers/ReadOnlySequenceStream.cs b/src/libraries/System.Memory/src/System/Buffers/ReadOnlySequenceStream.cs
index 9367aea898540a..0cdd942de199c8 100644
--- a/src/libraries/System.Memory/src/System/Buffers/ReadOnlySequenceStream.cs
+++ b/src/libraries/System.Memory/src/System/Buffers/ReadOnlySequenceStream.cs
@@ -8,7 +8,7 @@
 namespace System.Buffers
 {
     /// <summary>
-    /// Provides a seekable, read-only <see cref="Stream"/> over a <see cref="ReadOnlySequence{Byte}"/>.
+    /// Provides a read-only, non-seekable <see cref="Stream"/> for reading from a <see cref="ReadOnlySequence{Byte}"/>.
     /// </summary>
     /// <remarks>
     /// <para>The underlying sequence is not copied; reads are served directly from its segments.</para>
@@ -18,7 +18,6 @@ public sealed class ReadOnlySequenceStream : Stream
     {
         private ReadOnlySequence<byte> _sequence;
         private SequencePosition _position;
-        private long _absolutePosition;
         private bool _isDisposed;
         private CachedCompletedInt32Task _lastReadTask;
 
@@ -30,15 +29,19 @@ public ReadOnlySequenceStream(ReadOnlySequence<byte> source)
         {
             _sequence = source;
             _position = source.Start;
-            _absolutePosition = 0;
-            _isDisposed = false;
         }
 
         /// <inheritdoc />
         public override bool CanRead => !_isDisposed;
 
         /// <inheritdoc />
-        public override bool CanSeek => !_isDisposed;
+        // Keep this intentionally non-seekable: backward positioning requires traversing segments
+        // again from the beginning, making repeated seeks worst-case O(N). ReadOnlySequence<T>
+        // segment boundaries may be indirectly controlled by an untrusted network client through
+        // packet framing, so even correct stitching logic can produce adversarial fragmentation.
+        // Consumers must remain resilient against the worst technically compliant implementation
+        // rather than assuming ASP.NET-like segmentation.
+        public override bool CanSeek => false;
 
         /// <inheritdoc />
         public override bool CanWrite => false;
@@ -46,43 +49,16 @@ public ReadOnlySequenceStream(ReadOnlySequence<byte> source)
         private void EnsureNotDisposed() => ObjectDisposedException.ThrowIf(_isDisposed, this);
 
         /// <inheritdoc />
-        public override long Length
-        {
-            get
-            {
-                EnsureNotDisposed();
-                return _sequence.Length;
-            }
-        }
+        // Keep Length and Position unsupported to match the standard contract encoded by the
+        // stream conformance tests for streams where CanSeek is false, even though the underlying
+        // sequence can provide its length cheaply.
+        public override long Length => throw new NotSupportedException(SR.NotSupported_UnseekableStream);
 
         /// <inheritdoc />
         public override long Position
         {
-            get
-            {
-                EnsureNotDisposed();
-                return _absolutePosition;
-            }
-            set
-            {
-                EnsureNotDisposed();
-                ArgumentOutOfRangeException.ThrowIfNegative(value);
-
-                if (value >= _sequence.Length)
-                {
-                    _position = _sequence.End;
-                }
-                else if (value >= _absolutePosition)
-                {
-                    _position = _sequence.GetPosition(value - _absolutePosition, _position);
-                }
-                else
-                {
-                    _position = _sequence.GetPosition(value, _sequence.Start);
-                }
-
-                _absolutePosition = value;
-            }
+            get => throw new NotSupportedException(SR.NotSupported_UnseekableStream);
+            set => throw new NotSupportedException(SR.NotSupported_UnseekableStream);
         }
 
         /// <inheritdoc />
@@ -97,11 +73,6 @@ public override int Read(Span<byte> buffer)
         {
             EnsureNotDisposed();
 
-            if (_absolutePosition >= _sequence.Length)
-            {
-                return 0;
-            }
-
             ReadOnlySequence<byte> remaining = _sequence.Slice(_position);
             int n = (int)Math.Min(remaining.Length, buffer.Length);
             if (n <= 0)
@@ -111,7 +82,6 @@ public override int Read(Span<byte> buffer)
 
             remaining.Slice(0, n).CopyTo(buffer);
             _position = _sequence.GetPosition(n, _position);
-            _absolutePosition += n;
             return n;
         }
 
@@ -159,19 +129,18 @@ public override void CopyTo(Stream destination, int bufferSize)
             ValidateCopyToArguments(destination, bufferSize);
             EnsureNotDisposed();
 
-            if (_absolutePosition >= _sequence.Length)
+            ReadOnlySequence<byte> remaining = _sequence.Slice(_position);
+            if (remaining.IsEmpty)
             {
                 return;
             }
 
-            ReadOnlySequence<byte> remaining = _sequence.Slice(_position);
             foreach (ReadOnlyMemory<byte> segment in remaining)
             {
                 destination.Write(segment.Span);
             }
 
             _position = _sequence.End;
-            _absolutePosition = _sequence.Length;
         }
 
         /// <inheritdoc />
@@ -185,24 +154,23 @@ public override Task CopyToAsync(Stream destination, int bufferSize, Cancellatio
                 return Task.FromCanceled(cancellationToken);
             }
 
-            if (_absolutePosition >= _sequence.Length)
+            ReadOnlySequence<byte> remaining = _sequence.Slice(_position);
+            if (remaining.IsEmpty)
             {
                 return Task.CompletedTask;
             }
 
-            return CopyToAsyncCore(destination, cancellationToken);
+            return CopyToAsyncCore(remaining, destination, cancellationToken);
         }
 
-        private async Task CopyToAsyncCore(Stream destination, CancellationToken cancellationToken)
+        private async Task CopyToAsyncCore(ReadOnlySequence<byte> remaining, Stream destination, CancellationToken cancellationToken)
         {
-            ReadOnlySequence<byte> remaining = _sequence.Slice(_position);
             foreach (ReadOnlyMemory<byte> segment in remaining)
             {
                 await destination.WriteAsync(segment, cancellationToken).ConfigureAwait(false);
             }
 
             _position = _sequence.End;
-            _absolutePosition = _sequence.Length;
         }
 
         /// <inheritdoc />
@@ -218,43 +186,7 @@ private async Task CopyToAsyncCore(Stream destination, CancellationToken cancell
         public override ValueTask WriteAsync(ReadOnlyMemory<byte> buffer, CancellationToken cancellationToken = default) => throw new NotSupportedException(SR.NotSupported_UnwritableStream);
 
         /// <inheritdoc/>
-        public override long Seek(long offset, SeekOrigin origin)
-        {
-            EnsureNotDisposed();
-
-            long basePosition = origin switch
-            {
-                SeekOrigin.Begin => 0L,
-                SeekOrigin.Current => _absolutePosition,
-                SeekOrigin.End => _sequence.Length,
-                _ => throw new ArgumentException(SR.Argument_InvalidSeekOrigin, nameof(origin))
-            };
-
-            ArgumentOutOfRangeException.ThrowIfGreaterThan(offset, long.MaxValue - basePosition);
-
-            long absolutePosition = basePosition + offset;
-
-            if (absolutePosition < 0)
-            {
-                throw new IOException(SR.IO_SeekBeforeBegin);
-            }
-
-            if (absolutePosition >= _sequence.Length)
-            {
-                _position = _sequence.End;
-            }
-            else if (absolutePosition >= _absolutePosition)
-            {
-                _position = _sequence.GetPosition(absolutePosition - _absolutePosition, _position);
-            }
-            else
-            {
-                _position = _sequence.GetPosition(absolutePosition, _sequence.Start);
-            }
-
-            _absolutePosition = absolutePosition;
-            return absolutePosition;
-        }
+        public override long Seek(long offset, SeekOrigin origin) => throw new NotSupportedException(SR.NotSupported_UnseekableStream);
 
         /// <inheritdoc />
         public override void Flush() { }
diff --git a/src/libraries/System.Memory/tests/ReadOnlyBuffer/ReadOnlySequenceStream.ConformanceTests.cs b/src/libraries/System.Memory/tests/ReadOnlyBuffer/ReadOnlySequenceStream.ConformanceTests.cs
index 33f32fc05d662b..6e3dc733f4c28d 100644
--- a/src/libraries/System.Memory/tests/ReadOnlyBuffer/ReadOnlySequenceStream.ConformanceTests.cs
+++ b/src/libraries/System.Memory/tests/ReadOnlyBuffer/ReadOnlySequenceStream.ConformanceTests.cs
@@ -4,13 +4,12 @@
 using System.Buffers;
 using System.IO.Tests;
 using System.Threading.Tasks;
-using Xunit;
 
 namespace System.Memory.Tests
 {
     public class ROSequenceStreamConformanceTests : StandaloneStreamConformanceTests
     {
-        protected override bool CanSeek => true;
+        protected override bool CanSeek => false;
         protected override bool CanSetLength => false;
         protected override bool NopFlushCompletesSynchronously => true;
 
@@ -34,27 +33,6 @@ protected virtual ReadOnlySequence<byte> CreateSequence(byte[] data)
 
         protected override Task<Stream?> CreateReadWriteStreamCore(byte[]? initialData)
             => Task.FromResult<Stream?>(null);
-
-        public override async Task Seek_PastEnd_ReadReturns0(SeekMode mode)
-        {
-            await base.Seek_PastEnd_ReadReturns0(mode);
-
-            // ReadOnlySequenceStream-specific: seeking past end against an empty sequence
-            // is allowed, Position reflects the requested offset, and reads stay at 0.
-            var stream = new ReadOnlySequenceStream(ReadOnlySequence<byte>.Empty);
-            Assert.Equal(0, stream.Length);
-            Assert.Equal(0, stream.Position);
-
-            byte[] buffer = new byte[10];
-            Assert.Equal(0, stream.Read(buffer, 0, 10));
-
-            stream.Seek(0, SeekOrigin.Begin);
-            Assert.Equal(0, stream.Position);
-
-            long newPosition = stream.Seek(1, SeekOrigin.Begin);
-            Assert.Equal(1, newPosition);
-            Assert.Equal(1, stream.Position);
-        }
     }
 
     /// <summary>
diff --git a/src/libraries/System.Private.CoreLib/src/System/IO/ReadOnlyMemoryStream.cs b/src/libraries/System.Private.CoreLib/src/System/IO/ReadOnlyMemoryStream.cs
index 739a3391ff872c..82d05825286fda 100644
--- a/src/libraries/System.Private.CoreLib/src/System/IO/ReadOnlyMemoryStream.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/IO/ReadOnlyMemoryStream.cs
@@ -7,11 +7,8 @@
 namespace System.IO
 {
     /// <summary>
-    /// Provides a seekable, read-only <see cref="Stream"/> over a <see cref="ReadOnlyMemory{Byte}"/>.
+    /// Provides a seekable, read-only <see cref="Stream"/> for reading from a <see cref="ReadOnlyMemory{Byte}"/>.
     /// </summary>
-    /// <remarks>
-    /// <para>The stream cannot be written to. <see cref="CanWrite"/> always returns <see langword="false"/>.</para>
-    /// </remarks>
     public sealed class ReadOnlyMemoryStream : Stream
     {
         private ReadOnlyMemory<byte> _memory;
@@ -22,7 +19,11 @@ public sealed class ReadOnlyMemoryStream : Stream
         /// <summary>
         /// Initializes a new instance of the <see cref="ReadOnlyMemoryStream"/> class over the specified <see cref="ReadOnlyMemory{Byte}"/>.
         /// </summary>
-        /// <param name="source">The <see cref="ReadOnlyMemory{Byte}"/> to wrap.</param>
+        /// <param name="source">The memory region from which to create the stream.</param>
+        /// <remarks>
+        /// The existing contents of <paramref name="source"/> are immediately readable.
+        /// Clear rented or reused memory before constructing the stream if its existing contents should not be exposed.
+        /// </remarks>
         public ReadOnlyMemoryStream(ReadOnlyMemory<byte> source)
         {
             _memory = source;
diff --git a/src/libraries/System.Private.CoreLib/src/System/IO/StringStream.cs b/src/libraries/System.Private.CoreLib/src/System/IO/StringStream.cs
index 36fba65dc37a90..c7074545db9a3f 100644
--- a/src/libraries/System.Private.CoreLib/src/System/IO/StringStream.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/IO/StringStream.cs
@@ -9,7 +9,7 @@ namespace System.IO
 {
     /// <summary>
     /// Provides a read-only, non-seekable <see cref="Stream"/> that encodes a <see cref="string"/> or
-    /// <see cref="ReadOnlyMemory{Char}"/> into bytes on-the-fly using a specified <see cref="System.Text.Encoding"/>.
+    /// <see cref="ReadOnlyMemory{Char}"/> into bytes on-the-fly using a specified <see cref="Encoding"/>.
     /// </summary>
     /// <remarks>
     /// <para>This stream never emits a byte order mark (BOM). Callers who need a BOM can prepend it themselves.</para>
@@ -74,6 +74,8 @@ public StringStream(ReadOnlyMemory<char> text, Encoding encoding)
         public override bool CanRead => !_disposed;
 
         /// <inheritdoc/>
+        // Keep this intentionally non-seekable: backward positioning requires re-running the
+        // encoder from the beginning, making repeated seeks worst-case O(N).
         public override bool CanSeek => false;
 
         /// <inheritdoc/>
diff --git a/src/libraries/System.Private.CoreLib/src/System/IO/WritableMemoryStream.cs b/src/libraries/System.Private.CoreLib/src/System/IO/WritableMemoryStream.cs
index 802c6191c7501c..4f681578c36d57 100644
--- a/src/libraries/System.Private.CoreLib/src/System/IO/WritableMemoryStream.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/IO/WritableMemoryStream.cs
@@ -7,7 +7,7 @@
 namespace System.IO
 {
     /// <summary>
-    /// Provides a seekable, writable <see cref="Stream"/> over a <see cref="Memory{Byte}"/>.
+    /// Provides a seekable <see cref="Stream"/> for reading from and writing to a <see cref="Memory{Byte}"/>.
     /// </summary>
     public sealed class WritableMemoryStream : Stream
     {
@@ -20,7 +20,11 @@ public sealed class WritableMemoryStream : Stream
         /// <summary>
         /// Initializes a new instance of the <see cref="WritableMemoryStream"/> class over the specified <see cref="Memory{Byte}"/>.
         /// </summary>
-        /// <param name="buffer">The <see cref="Memory{Byte}"/> to wrap.</param>
+        /// <param name="buffer">The memory region from which to create the stream.</param>
+        /// <remarks>
+        /// The existing contents of <paramref name="buffer"/> are immediately readable.
+        /// Clear rented or reused memory before constructing the stream if its existing contents should not be exposed.
+        /// </remarks>
         public WritableMemoryStream(Memory<byte> buffer)
         {
             _memory = buffer;
diff --git a/src/libraries/System.Runtime/tests/System.IO.Tests/StringStream/StringStreamTests_String.cs b/src/libraries/System.Runtime/tests/System.IO.Tests/StringStream/StringStreamTests_String.cs
index b123499b6132ca..28b5a2d4077e40 100644
--- a/src/libraries/System.Runtime/tests/System.IO.Tests/StringStream/StringStreamTests_String.cs
+++ b/src/libraries/System.Runtime/tests/System.IO.Tests/StringStream/StringStreamTests_String.cs
@@ -70,10 +70,15 @@ public void ThrowsOnNullEncoding()
         [Fact]
         public void StreamCapabilities()
         {
-            var stream = new StringStream("test", Encoding.UTF8);
+            using var stream = new StringStream("test", Encoding.UTF8);
+
             Assert.True(stream.CanRead);
             Assert.False(stream.CanSeek);
             Assert.False(stream.CanWrite);
+            Assert.Throws<NotSupportedException>(() => stream.Length);
+            Assert.Throws<NotSupportedException>(() => stream.Position);
+            Assert.Throws<NotSupportedException>(() => stream.Position = 0);
+            Assert.Throws<NotSupportedException>(() => stream.Seek(0, SeekOrigin.Begin));
         }
 
         [Fact]